1. Field of the Invention
The present invention generally relates to photoresist processing and the use of a kernel function to characterize the photolithography process for the purpose of reducing corner rounding and line-end foreshortening and more particularly relates to a method and system for identifying the range and shape of the kernel function based upon the difference between the photolithographic mask and the resist pattern.
2. Description of the Related Art
Photolithography is the technology of reproducing patterns using light. As presently used in the semiconductor industry, a photomask pattern for a desired circuit is transferred to a wafer through light exposure, development, etch, and resist strip, etc. As the feature sizes on a circuit become smaller and smaller, the circuit shape on the wafer differs from the original mask pattern more and more. In particular, corner rounding (see FIGS. 1, 2a, and 2b, discussed below), line-end foreshortening (see FIG. 2c, discussed below), different print biases between narrow and wide lines (see FIG. 3, discussed below), etc. are typically observed. These phenomena are called optical proximity effects.
One of the main reasons for optical proximity effects is light diffraction, resulting in the dependency of the aerial image of a feature on its surroundings. Moreover, the final photoresist image depends on both the aerial image from optical imaging process and the subsequent etch and photoresist development processes. Therefore, there is a need for a characterization of the whole lithography processes, including optical imaging process and subsequent etch and photoresist development processes. Such a characterization would provide the basis for correcting optical proximity effects and for improving photomask printability. Such a characterization would account for all aspect of the photolithographic process including the effects of the imaging light wavelength, the numerical aperture of the imaging projection system, the partial coherence factor "sgr", and the specific photoresist used.
It is, therefore, an object of the present invention to provide a structure and method for determining a range and a shape of a kernel function of a lithographic system which includes exposing, in the lithographic system, a photosensitive layer on a top surface of a substrate through a mask having a mask image, the mask image being of sufficient width to ensure a transferred image will not exhibit foreshortening but will exhibit corner rounding; developing the photosensitive layer to form the transferred image in the photosensitive layer; measuring a distance from an intersection of projected extensions of edges of the transferred image to a point along one edge where corner rounding starts; and defining the range of the kernel function as the measured distance. The projected extension edges are an unaltered version of the mask image overlaid on the transferred image and the foreshortening is a reduction in length of transferred images when compared to the mask image. Corner rounding occurs as a result of light diffraction and photosensitive layer development processes. The measuring simultaneously accommodates for the light diffraction and photosensitive layer development effects.
The invention can also comprise a method of determining a range and a shape of a kernel function of a lithographic system which includes exposing, in the lithographic system, a photosensitive layer on a top surface of a substrate through a mask having a plurality of mask images of decreasing widths; developing the photosensitive layer to form a plurality of corresponding transferred images in the photosensitive layer; comparing widths of the transferred images with widths of corresponding mask images; and defining the range of the kernel function as a smallest transferred image width that equals a corresponding mask image width. The comparing includes overlaying unaltered versions of the mask images on corresponding ones of the transferred images, wherein all transferred images having widths below the range of the kernel function are smaller than corresponding mask images. The difference between a mask image width and a corresponding transferred image width occurs as a result of light diffraction and photosensitive layer development processes, wherein comparing simultaneously accommodates for the light diffraction and photosensitive layer development effects.